Electrical utilities must burn increasing quantities of fossil fuels to satisfy the ever-increasing demand for electric power. At the same time, electric utilities face increasing clean-air standards that are imposed upon their operation. In trying to satisfy the divergent demands of increasing power and decreased air pollution, electrical utilities have turned to using low-sulfur coals to fire their boilers and generate the steam needed for electrical power generation.
Electrical utilities have long relied upon electrostatic means such as electrostatic precipitators to remove particulate matter from boiler flue gas. The efficiency of operation of the electrostatic precipitators in the removal of particulate matter from boiler flue gas is dependent, in part, upon the electrical resistivity of the entrained particulate matter in boiler flue gas. It has been found that where a boiler is fired with low sulfur content, the entrained particulate matter in the boiler flue gas has a high resistivity, for example, 10.sup.13 ohm-cm resistance and more, the most efficient removal of particulate matter by electrostatic precipitation requires that its resistivity be substantially lower; for example, on the order of about 10.sup.8 ohm-cm. When the resistivity of the particulate matter is high, for example, on the order of 10.sup.13 ohm-cm, the efficiency of electrostatic precipitation is substantially reduced. Thus, reduced efficiency in the operation of electrostatic precipitators with the flue gas from low-sulfur coals as a result of the higher resistivity of its flue gas particles can offset the reduced or potentially reduced air pollution sought through the use of the more expensive low-sulfur coals.
One solution to this problem has been to condition the boiler flue gas prior to its entrance into the electrostatic precipitator by the use of a conditioning agent to reduce the resistivity of the entrained particles within the boiler flue gas. Among the various chemicals which have been used as conditioning agents for boiler flue gas are water, anhydrous ammonia and various ammonia-bearing solutions, sulfuric acid, phosphoric acid and most commonly sulfur trioxide.
Sulphur trioxide flue gas-conditioning systems have included systems which store liquefied sulfur which is fed to a sulfur burner in which the sulfur is converted by combustion predominantly to sulfur dioxide. The systems then pass the sulfur dioxide to a catalytic converter which employs a vanadium pentoxide catalyst to convert the sulfur dioxide into sulfur trioxide. The sulfur trioxide created by such systems is piped to a nozzle system for injection into ducts carrying the boiler flue gas and its entrained particulate material to reduce the electrical resistivity of the flue gas particulate matter for removal by an electrostatic precipitator.
Sulfur trioxide conditioning systems have been proposed to condition the flue gas streams from more than one boiler with sulfur trioxide from a single conditioning agent source. For example, U.S. Pat. No. 4,333,746 proposes a flue gas conditioning system including a catalytic converter whose output is divided and connected through a system of conduits and control valves with two flue gas conduits carrying the flue gas from two boilers, and further including a separate purging system of conduits and control valves to direct a flow of purging air within the system.
The prior proposals for such multi-boiler flue gas conditioning systems have not been satisfactory. Individual boilers must be free to, and are likely to operate at, different capacities because of differing and variable electrical load requirements, operating problems or outages and the like, and each of their electrostatic precipitators face varying flue gas flow and particle conditions and must operate at high efficiencies at all times. A multi-boiler flue gas conditioning system must be able to provide separate, independently controlled flows of conditioning agent into each of the plurality of variable flue gas flows from the multiple boilers and accurately respond to varying load signals from the boilers, the turbines or their controls and provide variations needed to maintain high precipitator operating efficiencies and low flue gas opacities in the plurality of flue gas streams from the boilers. In addition, if one boiler must be shut down because of a forced power outage, the conditioning system must be able to enter a purge cycle to clear sulfur trioxide from the portion of the system carrying sulfur trioxide to the flue gas of the shut-down boiler without significantly altering the flow of conditioning agent to the other operating boilers.
In prior proposed multi-boiler flue gas conditioning systems, such as that proposed in U.S. Pat. No. 4,333,746, the catalytic converter providing the conditioning agent must be designed to provide the total maximum flow of conditioning agent for all of the boilers to which it is connected, and a separate additional air blower and heater must be provided for purging the system. Furthermore, the catalytic converter and the other apparatus providing sulfur dioxide to the catalytic converter have generally been assembled as a unit for shipping installation and operation, and the assembled SO.sub.3 -producing unit is not capable of location near multiple flue gas conduits leading from multiple boilers so that, in such prior multi-boiler systems, sulfur trioxide conditioning agent had to be carried through lengthy conduits and frequently over tortuous paths from the SO.sub.3 -producing unit to the injectors of the various flue gas conduits of the multiple boilers, thus requiring both extended warm-up times prior to the introduction of sulfur trioxide to remote flue gas conduits, because of the high acid dew point of SO.sup.3 (550.degree. F.-288.degree. C.), and extended system purge times upon shut down of a boiler unit, and increased difficulty in purging long tortuous conduits of sulfur trioxide.
In addition, sulfur trioxide conditioning agent can be made at a single flow rate at any one time and the resulting single flow of sulfur trioxide must be divided among the multiple boilers depending upon the boiler load of each of the boilers, and the characteristics of the particulate matter generated by each of the boilers and of operation of each of the electrostatic precipitators; no successful system for such operation is known. Providing sulfur trioxide at a rate that is too low or too high to any one of the boiler conduits can result in ineffective operation of its electrostatic precipitator, and excessive stack emission, corrosion of system components and a blue acid plume.
Because of the problems above and others, prior multi-boiler flue gas conditioning systems have been inefficient, inflexible, expensive and generally ineffective. The multi-burner flue gas conditioning systems that have been tried have proven to be unsatisfactory in service and many have been abandoned. As a result, such multi-burner flue gas conditioning systems are no longer being offered and flue gas conditioning system suppliers are suggesting a separate flue gas conditioning system for each boiler flue gas stream.